1. Field of the Invention
The present invention generally relates to nuclear imaging. In particular, the present invention relates to systems and methods for pinhole collimator imaging.
2. Description of the Background Art
The ability of nuclear medicine modalities to provide physiologic functions in vivo, the metabolism of several substrates, and the binding potential of particular receptors of the cardiovascular system has brought incremental value to nuclear cardiology and has broadened the clinical relevancy of the modality.
Many treatment algorithms in cardiology use the left ventricle ejection fraction as an initial measure in clinical decision making. Noninvasive myocardium perfusion imaging should guide the physician in planning the appropriate management of patients with known or suspected coronary artery disease. In addition, dynamic (quantitative) cardiac imaging provides quantification in particular physiologic processes and biochemical pathways of interest based on kinetic modeling.
However, despite a relatively long history of cardiac Single Photon Emission Computed Tomography (hereinafter referred to as SPECT) imaging, greater radiotracer availability and longer-lived isotopes, dynamic imaging is generally regarded as the exclusive domain of positron emission tomography (PET), and only limited attempts have been made to extract quantitative physiologic parameters using SPECT. The main limitation of dynamic SPECT is its inferior detection efficiency and spatial resolution as compared with PET.
Other challenging aspects of imaging cardiovascular SPECT are the anatomical location of the heart, bulky detectors and the movement of the myocardium.
The heart is located in the middle of the chest behind the breastbone, between the lungs, and rests in a moistened chamber called the pericardial cavity which is surrounded by the ribcage. Further, a layer of muscle lies below the heart. As a result, the heart is well protected, but difficult to image. In addition, conventional gamma cameras are heavy and bulky because they have a very large single crystal, typically 40×60 cm2, coupled to a heavy collimator so that it has limited access to the body contour and view angle.
The most common way to address the movement problem in existing devices is a procedure called MUGA (multiple gated acquisition) that involves gating of the image acquisition with R-wave signals from an EKG and generating time-slice images per cardiac cycle. To avoid motion blur, a high count rate capability is typically recommended.
Existing SPECT imaging allows for multiple 2-D images to be taken from different angles then recreated using a SPECT computer program to produce a 3-D image.
Further, existing SPECT imaging creates an image utilizing a scintillator. A scintillator is a material that has the ability to absorb a photon and convert that energy into light. Scintillators are used to detect the energy given off by a radioactive isotope. Existing scintillators should be able to convert much of the incident energy to light. Existing scintillators can be either organic or inorganic with each having their own benefits depending on the intended use.
Additionally, existing SPECT imaging also uses collimators to limit the direction of impinging photons as they approach the scintillator. Generally, existing collimators are made out of lead, tungsten, or copper-beryllium. There are two principal types of collimators used in medical imaging. The pin-hole collimator is primarily used in studying very localized objects such as a gland or other organ. It consists of a dense material with a single small hole drilled in the middle. Pin-hole collimators offer the benefit of high magnification of a single object, but lose resolution and sensitivity as the field of view gets wider. On the other hand, a parallel-hole collimator consists of a large number of holes drilled or etched into the material that accept photons only moving perpendicular to the scintillator.